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SMD solder joint calculation

#18659

SMD solder joint calculation | 16 January, 2002

Hi...

We tend to do a lot of footprint calculating for new SMD components. But what about the amount of solder paste on the solder pads.

The amount of solder alloy is of cause determined by the stencil aperture size, stencil thickness and the solder paste metal %. And, we all know the ideal solder joint shape. But how can we reach this, if we don�t take the solder amount into account when calculating the footprint sizes ! Of cause there is a big difference in the shape of the solder joint shapes for a chip or a IC footprint. So there must be different formulas used for calculating the correct solder alloy volume !

Does anyone know of a fairly easy way of calculating the necessary amount of solder paste to form a proper solder joint for different component types ??

Brian

www.smtinfocus.com

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Dason C

#18661

SMD solder joint calculation | 16 January, 2002

You can use the IPC-7525, Stencil Design Guidelines for start up.

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Terry Burnette

#18667

SMD solder joint calculation | 16 January, 2002

Terry Burnette

#18668

SMD solder joint calculation | 16 January, 2002

In his book titled "Manufacturing Techniques for Surface Mount Assemblies" author R.J. Klein Wassink describes the relationship between solder land dimensions, component lead coplanarity, board warpage, solder volumes, and solder paste flux wetting behaviors. Mr. Wassink provides the following formula for calculating the necessary solder volumes for a given lead coplanarity: (Wp/W) x d x Hs x f(sh) = finished solder height

Wp = Solder paste stencil width W = PCB solder pad width d = Stencil thickness Hs = Volume fraction of solder in paste ( Hs is about 55% with 90% solder metal) f(sh) = Shape factor = the height h of the circle segment with cord of W divided by the thickness d1 of the rectangle with the same area and same length W. The factor is about 1.4. This formula is used to calculate finished solder height which when used with Wassink's rule, "that a maximum non-coplanarity of less than 75% of the finished solder height, on the copper land provides a condition for stable and robust processing" will show how much non-coplanarity your process can handle. For a .020" pitch QFP, which uses a PCB solder pad width of .012", stencil width of .011", and stencil thickness of .006", the calculations show the following:

.011/.012 x .006 x .55 x 1.4 = .0042" finished solder height. .0042" x .75 = .0032" max. lead non-coplanarity for robust processing.

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#18673

SMD solder joint calculation | 16 January, 2002

No, not as such. Consider: * Using the IPC calculator [free at http://www.ipc.org]. It allows you to consider / assess the amount of solder when designing pads. * Reviewing IPC-7525 - Stencil Design Guidelines. It gives general aperture guidelines. * Reading �Manufacturing Techniques For Surface Mounted Assemblies� RJ Klein Wassink & MMF Verguld; Electrochemical Publications [0901150304]. It gives pinching guidelines for different pad sizes. * Employing the �Evolver� model [ http://www.ctcms.nist.gov/programs/solder/archive_solder.html ]. It is used to predict bridging. * Performing an experiment like http://www.msl.com/technical_capabilities/full_solderpaste.html

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